The present invention relates generally to articles used in conspicuity applications such as highway signs, and has particular application to situations it) where an off-axis stationary light source is used to illuminate such a sign.
The use of retroreflective sheeting for signing applications is known. "Retroreflective" as used herein refers to the attribute of reflecting an incident light ray in a direction antiparallel to its incident direction, or nearly so, such that it returns to the light source or the immediate vicinity thereof. Known sheeting constructions use miniature glass beads in connection with reflective coatings, or alternatively cube corner arrays, to retroreflect incident light. They are designed to provide specified brightness values for a range of entrance angles. For design purposes, a typical angular separation of the driver of a vehicle and the vehicle's headlamps is taken to be less than two degrees; many sheeting constructions specify retroreflective brightness at a narrow 0.2 degree observation angle. The terms "observation angle" and "entrance angle" are defined along with other related terms at the end of the detailed description section.
FIG. 1A depicts a typical situation where a vehicle approaches a highway sign 2 positioned at the side of the road. A portion 4 of the light emitted from a headlamp impinges at an entrance angle .beta. on retroreflective sheeting 6 disposed on the face of the sign 2. Sheeting 6 can be, for example, one of a variety of Scotchlite.TM. brand reflective sheetings available from 3M Company, such as "Engineer Grade" or "High Intensity Grade" sheeting. The sheeting 6 retroreflects the incident light in a narrow cone 8 which includes the driver's eye 10. The cone 8 has an angular half width 12, measured from the central maximum out to 10% of maximum brightness, of about 1.7 degrees for standard "Engineer Grade" sheeting and about 0.75 degrees for standard "High Intensity Grade" sheeting. As the vehicle advances along the direction 14 of the road, the entrance angle .beta. increases and the cone remains centered on the vehicle headlamp. Because the retroreflected light is confined to a relatively narrow cone, the perceived brightness of the sign can be relatively high, depending upon the angular proximity of the observer's eye to the light source.
FIG. 1B depicts an alternative arrangement similar to that disclosed in PCT, Publication WO 96/04638. A sign 2 is illuminated by a stationary light source 16, which is positioned at an entrance angle of about 0 to 30 degrees relative to portions of the sign. Retroreflective sheeting 18 on the sign face reflects light in a wide cone, defined by an observation angle ranging from 0 to about 40 degrees. The cone of reflected light is wide enough to include an observer or driver 10 traveling along the roadway direction 14.
The present application discloses articles which can be used to advantage in arrangements like that of FIG. 1B, while maintaining desirable retroreflective properties as in FIG. 1A. As shown in FIG. 1C, light 4 from light source 16 is incident at an entrance angle .beta. on a reflective article 20. The reflective article 20 redirects the light preferentially into two beams 22,24. The article 20 is designed such that one ol the beams 24 is directed towards and fills an observation zone 26. The light source 16, of conventional design, is located outside of the observation zone. Efficiency is enhanced by reducing wasted light, thereby increasing the amount of light available to illuminate the observation zone. The brightness of the observed bean 24 relative to the nonobserved beam 22 can be enhanced by using reflective elements in the article which have highly nonorthogonal reflecting surfaces in contrast to conventional cube corner elements.